1. Field of the Invention
This invention relates to mufflers, as used for example on motor vehicles, and, more particularly, to a silencing member which is incorporated into the mufflers. The invention is also directed to a method of manufacturing the silencing member.
2. Background Art
It is conventional to use a pre-muffler or a sub-muffler on the upstream side of an exhaust system for an internal combustion engine used, for example, in a motor vehicle. Since the pre-muffler/sub-muffler is disposed behind a purging device upstream of a main muffler, high temperature exhaust gas flows at high speed through an exhaust pipe of the pre-muffler/sub-muffler.
A conventional pre-muffler/sub-muffler is shown at 10 in FIG. 18. Stainless wool or stainless wool rope 12 is wrapped around an inner exhaust pipe 14. A multitude of tiny holes 16 are formed through the exhaust pipe 14. Glass wool 18 is wound around the stainless wool/stainless wool rope 12. The glass wool 18 is surrounded by an outer pipe 20. The glass wool 18 functions as a silencing medium. The stainless wool/wool rope 12, which has higher thermal resistance than the glass wool 18, prevents the glass wool 18 from being drawn into the exhaust pipe 14 through the holes 16 and scattering therewithin.
It is also known in the art to use glass cloth with glass fibers made into a plain weave fabric instead of the stainless wool/wool rope 12.
In FIG. 19, a conventional muffler is shown at 22 and has a housing 24 with an end plate 26. Openings 28 are formed through the end plate 26. Long, glass wool fibers 30 are directed through the openings 28 into a space 32 formed between an inner pipe 34, with holes 36 therethrough, and an outer pipe 38 that is part of the housing 24.
The stainless wool/wool rope 12 is relatively expensive and generally its use in this environment requires special processing steps. Scattering of the glass wool 18 within the exhaust pipe 14 is more effectively prevented by increasing the amount of stainless wool/wool rope 12 that is used. By increasing the amount of stainless wool/wool rope 12, the overall cost of manufacture rises.
With the conventional use of glass wool, the glass is woven into a plain weave fabric or a twill weave fabric which has a fine texture and high permeability. The glass captures soot, or the like, entrained in the exhaust gas and eventually becomes clogged. As a result, the silencing effect of the glass wool may be diminished. Further, plain weave and twill weave fabrics, made from glass, may be inefficient and costly to produce.
The manufacturing method described with respect to FIG. 19 reduces costs over the method described with respect to FIG. 18. However, the glass fibers 30 introduced as shown in FIG. 19, may ineffectively fill the space 32. That is, the density of the fibers 30 may be less than desired. Further, there may be an uneven distribution of the fibers 30 within the space 32. If the space 32 is insufficiently filled with the fibers 30, carbon contained in exhaust gas may migrate to within the gaps between the fibers 30 and may adhere thereto. The accumulated and stagnant carbon may diminish the acoustic absorption effect. Further, the accumulated carbon conducts and retains heat, potentially causing the glass fibers 30 to harden, deteriorate, and scatter into the inner pipe 34.
Further, the pressure of the exhaust gas in the inner pipe 34 rises and falls cyclically with the associated internal combustion engine operating. The glass fibers 30 subjected to this varying pressure tend to constantly shift within the housing 24. This movement may cause the glass fibers 30 to break and move through the holes 36 in the inner pipe 34.